LTE supports assistance signaling for wireless devices, also known as User Equipment (UE), capable of Cell-specific Reference Signal interference cancellation (CRS-IC). Wireless devices using Network Assisted Interference Cancellation and Suppression (NAICS) techniques may typically include advanced, often nonlinear, receivers that cancel and/or suppress transmissions from interfering cells. Such wireless devices determine interference parameters in order to use their advanced reception capabilities. The parameters are determined through the assistance signaling and/or by making estimates of (i.e., ‘blindly detecting’) the interference parameters. The complexity of blind detection generally increases (and the reliability of blind detection degrades) with the number of parameters and the number of possible values of the parameters.
Dynamic point selection (DPS) may also be supported for NAICS wireless devices. In this case, it can be desirable to identify the transmitted Physical Downlink Shared Channels (PDSCHs) in a transmission point (TP) specific manner. Transmission Mode 10 (TM10) wireless devices are configured with Demodulation Reference Signal (DMRS) identifiers (that is, nIDDMRS,i, also called configurable cell IDs, or “CCID”s herein) that identify the PDSCH intended for the wireless device. Given the similarity of this TP-specific association of CCIDs to cell IDs, a reasonable number for the points that the wireless device should track is the number of cells that are tracked in Rel-11, which is eight. With two nSCID values, then the maximum number of CCIDs the wireless device should be provided NAICS assistance for should be 2*8=16. Therefore, NAICS wireless devices may have to blindly detect up to 16 different CCIDs unless additional information can be used to exclude candidate CCIDs for NAICS reception.
LTE TPs may use the same physical cell ID (‘shared cell’ operation) or different cell IDs. When Dynamic Point Section (DPS) is used in shared cell operation, a wireless device can still receive a PDSCH because the CCID is used to identify the PDSCH, rather than the cell ID. Therefore NAICS assistance signaling cannot rely only on the physical cell ID to associate interferer parameters with the interfering transmitter. Accordingly, even where the physical cell ID, number of CRS antenna ports and MBSFN subframe configuration parameters for one or more neighbor cells is known, it is insufficient assistance for NAICS wireless devices that cancel LTE physical channels with rapidly varying information content such as a PDSCH with varying spatial multiplexing rank or PDSCH transmitted using a precoding derived from CSI feedback.
NAICS wireless devices that cancel interferers generally demodulate the interfering physical channel. This implies that they should be able to determine which parts of an interfering transmission contain the physical channel, as opposed to e.g. reference signals. In LTE Rel-11, physical downlink shared channels and cell-specific reference signals (CRS-RS) are transmitted in different resource elements on a given cell or TP, and so NAICS Wireless devices should know the CRS configuration of the interferer they are attempting to cancel. Similarly, TM9 and TM10 PDSCHs do not occupy resource elements carrying CSI-RS from the same cell or TP, and so in some cases it can be beneficial for a NAICS wireless device to know the CSI-RS configuration of a TM9 or TM10 interferer.
Wireless devices receiving TM10 PDSCHs can be configured to receive non-zero power (NZP) CSI-RSs and zero power (ZP) CSI-RSs. While a Rel-11 wireless device may be configured to receive 3 NZP CSI-RSs, only one NZP CSI-RS is likely to be transmitted from a given TP. The TP will generally not transmit in Resource Elements (REs) corresponding to the other two NZP CSI-RSs, in order to avoid interfering with the CSI-RSs transmitted by the corresponding two TPs. Therefore, if a wireless device is to know which resources are allocated for CSI-RS transmission in an interfering transmission, it will need to know both the NZP and ZP CSI-RS configurations. Furthermore, knowledge of the NZP CSI-RS configuration of the interferer does not guarantee that a wireless device will know if the interferer will actually transmit a CSI-RS.
An existing NAICS signaling proposal includes signaling Non-Zero Power (NZP) CSI-RS and Zero Power (ZP) CSI-RS information, and discusses that such signaling can allow a wireless device to find a strongest interfering TP, suggesting that a wireless device could do DMRS detection for a smaller set of IDs, and implying reduced wireless device complexity could result. However, the proposed techniques do not describe specifics of the signaling, including how parameters are signaled together, or details of how CRS and CSI-RS relate to parameters describing interfering PDSCH configurations. Furthermore, the proposal does not provide efficient signaling solutions addressing NZP CSI-RSs that may be blanked by a serving transmission point. Finally, it does not address signaling for improved TP power ranking schemes using a power offset for an NZP CSI-RS, nor how to control when a wireless device may use a CSI-RS for power ranking.
Measurements of CSI-RS for Radio Resource Measurement (RRM) purposes (often called ‘CSI-RSRP’) have been proposed. However, CSI-RSRP measurements do not indicate what the scheduled PDSCH power may be, since they are used for RRM and are not used for CQI feedback. Furthermore, NZP CSI-RS resources are not associated with a transmission of a particular PDSCH (identified by a DMRS scrambling ID), and so a Rel-11 wireless device can't associate power measurements made on the NZP CSI-RS with the power that could be used when receiving using its DMRS scrambling ID.
Instead of blindly detecting interfering PDSCH precoding matrices and rank, the wireless device may determine this information by decoding the PDCCH of an interfering cell. However, this requires the interfering cell to transmit PDCCHs to its served wireless devices such that NAICS wireless devices in neighbor cells can decode the PDCCHs, which may require extra resources (higher PDCCH aggregation levels or more transmit power). Furthermore, the effort to decode one or multiple neighbor PDCCHs on one or more interfering cell may significantly increase the complexity of a NAICS wireless device and/or the power the wireless device needs in NAICS operation.